1. Technical Field
The invention relates to a device for measuring the thickness of a measurement object, the same for example having the shape of a sheet or cargo goods, in a measuring gap, by means of or via a measuring mechanism attached on a machine frame, wherein the measuring mechanism has one or more distance measurement sensors oriented toward the measurement object, for the purpose of measuring the thickness thereof.
In addition, the invention relates to a method for measuring the thickness of a measurement object, the same for example having the shape of a sheet or cargo goods, in a measuring gap, by means of or via a measuring mechanism attached on a machine frame, wherein the measuring mechanism orients one or more distance measurement sensors toward the measurement object, for the purpose of measuring the thickness thereof.
2. Description of Related Art
Devices and methods in the class have been known for years in practice, in many different embodiments. In the known devices and methods, a continuous thickness measurement is performed, for example on sheet materials, using a C- or O-shaped measurement mechanism and contactless measurement methods. For the purpose of measuring thickness in this case, optical sensors (triangulation sensors, confocal measuring techniques), eddy current sensors, capacitive sensors, ultrasound sensors, radiometric sensors (beta emitters), etc. are used. The thickness measurement typically takes place using two distance measurement sensors from both sides, against the material surface of a measurement object. The difference of the two measurement reading signals gives the thickness of the measured material if the distance between the two distance measurement sensors is known. When sheet material is the object being measured, traversing measurement methods are used, wherein both distance measurement sensors are moved as a pair transversally to the direction in which the sheet material is being transported.
FIG. 1 shows a schematic illustration of the basic principle of differential thickness measurement and/or calibration, by means of two distance measurement sensors and a sensor- and calibration arrangement. The thickness d of a measurement object is determined by means of two distance measurement sensors 1 and 2, as thickness sensors, in that the distance of the two distance measurement sensors 1 and 2 from each other—the measurement gap dA—is detected by means of a calibration part 3 with the predefined thickness dK (calibration part thickness) according todA=dM1+dM2+dK and subsequently the sum of the two measurement values dM1 and dM2 is subtracted from the measurement gap dA in a routine operation:d=dA−(dM1+dM2)
The distance measurement sensors are integrated into C- or O-shaped measurement mechanism frames and are used in the industrial field. If no further precautions are taken in the construction of a device for measuring thickness, thermal changes in the surroundings affect the mechanical structure, particularly the measurement gap, dA and therefore the measurement result d.
In the case of small measurement mechanisms, the measurement mechanism can be made of a temperature-stable material. In the case of thickness measurement for cargo goods, the calibration part can be measured after each routine measurement, in order to eliminate the effect of the changes in the measurement gap.
However, in the case of thickness measurement for wide, sheet-shaped materials, particularly coils, inner liners, etc., the predefined distance between the two measurement sensors is critical. Particularly for the measurement of metallic strips in widths up to 1-2 m, or paper webs up to 6 m in width or more, it is particularly problematic that C-brackets and/or O-frames cannot be held in a mechanically stable manner over the width of the sheet material serving as the measurement object. As a result, the distance between the distance measurement sensors is not constant. Thus, when measurement precision in the micrometer range is required, even the smallest mechanical changes have an influence on the measured thickness reading. Such mechanical changes can occur as a result of vibrations, for example, or as a result of a longitudinal expansion due to temperature changes. As such, the distance between the distance measurement sensors is altered due to vibration or longitudinal expansion of the machine frame caused by thermal input. As an example, in the case of a machine frame made of V2A steel, with an expansion coefficient of 16 ppm/K, the distance between the two distance measurement sensors, given a change in temperature of 10° C. and a distance of 1 m between the distance measurement sensor mounted on an upper belt and the distance measurement sensor mounted on a lower belt, would change by 160 μm. As a result, for thickness measurements which commonly require precision within micrometers, it is not possible to make a precise measurement, due to the action of vibration and/or temperature. In the case of aluminum beams, which are frequently used in practice, an even larger change in distance occurs, because aluminum has a coefficient of expansion of 23 ppm/K.
To measure wide, sheet-like materials, particularly coils, inner liners, etc., a temperature-stable measurement mechanism could possibly be made exclusively using temperature-stable materials. However, the use of vibration-resistant or temperature-stable O-frames and/or C-frames, to prevent imprecision in measurement, demands a great deal of mechanical complexity and special materials, such as Invar®, for example, which results in considerable costs. In the measurement tasks indicated above, the distance measurement sensors would need to be moved over the material using linear axles. In addition, it is only possible to measure the calibration part in short intervals outside of the material sheet. As a result, it is not possible to detect and compensate a change in the measurement gap over the material.
In practice, a C-frame is known, wherein the distance measurement sensors are mounted on a glass plate having a minimal thermal expansion coefficient. However, this embodiment has the significant disadvantage that, in the case of a C-frame with long sides, the installation of the measurement sensors requires a great deal of time and effort. In addition, in the design using glass, it is very problematic that glass can brake very easily and is therefore not suited for use in rough industrial production applications.
In DE 42 20 501 A1, a method and a device are known for the optical thickness measurement of a sheet-like material, wherein for the purpose of avoiding mechanical complexity, the relative distance of each of the two distance measurement sensors is determined by means of a laser point projector. This laser point projector projects a laser beam in the direction of the movement of the distance measurement sensors. The two laser point projectors are attached in a temperature-stable and vibration-resistant holder made of Invar®, such that the relative positions of the laser sources with respect to each other remain as constant as possible. However, the known device has the problem that the optical sensors are not suited for industrial use in what is typically a rough production environment, because the optical sensors are subjected to vibrations, shock, contamination, etc. in this environment. In addition, it is difficult to precisely orient the lasers. A further disadvantage of the known device is that, as a result of the lasers being mounted on one side, even the smallest movements result in a misalignment of the beams, and this misalignment becomes larger as the sensors become farther from the beam source.